As one of the most important classes of heterocycles, pyrroles are not only important building blocks in the synthesis of natural products, but also key structural units in compounds with interesting biological activities. Pyrroles have also found broad application in materials chemistry. Accordingly, substantial attention has been paid to develop efficient methods for their synthesis. One of the common approaches to pyrrole synthesis is the Paal-Knorr reaction, in which 1,4-dicarbonyl compounds are converted into pyrroles by acidmediated dehydrative cyclization. However, this approach is usually subject to significant limitations in terms of substituents introduced, substitution patterns, or regioselectivities.
Although several novel synthetic strategies have been described in recent years, [for metal-catalyzed pyrrole syntheses see e.g. J. T. Kim, J T, et al., Angew. Chem. Int. Ed. (2003) 42, 98; Balme, G, Angew. Chem. Int. Ed. (2004) 43, 6238; St. Cyr, D J, et al., Org. Lett. (2007) 9, 449; Binder, J T, & Kirsch, S F, Org. Lett. (2006) 8, 2151; Gorin, D J, et al., J. Am. Chem. Soc. (2005) 127, 11260; Dhawan, R, & Arndtsen, B A, J. Am. Chem. Soc. (2004) 126, 468; Yamamoto, Y, et al., J. Am. Chem. Soc. (2005) 127, 10804; for metal-free pyrrole syntheses see e.g. Bullington, J L, et al., J. Org. Chem. (2002) 67, 9439; Attanasi, O A, et al., J. Org. Chem. (2002) 67, 8178; Shindo, M, et al., Org. Lett. (2007) 9, 1963; St. Cyr, D J, Am. Chem. Soc. (2007) 129, 12366] a general facile and regioselective approach to generate pyrroles with a wide functional group tolerance from readily available precursors is still lacking.
Accordingly, it is an object of the present invention to provide a process that allows a simple formation of pyrrole compounds under mild conditions.